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Blue Crabs Under Siege, A Crabs Warning.

By James Holland, Altamaha Riverkeeper

The blue crab (Callinectes sapidus) can be found in most waters of relatively low salinity along most of the eastern seaboard and along the shores of the Gulf of Mexico. Blue crabs have a life span of two to three years and occasionally one may live to be as much as four years old. The female blue crab (sooks) will spawn from 1/2 million up to approximately two million eggs in each spawn cycle. After the eggs hatch it can take up to eighteen months for the young to reach maturity.

Research shows that the mortality rate for blue crabs is approximately 98%. In other words only about 2% are believed to reach the adult stage of life.

During the life cycle from larva to juvenile on into adulthood the young blue crabs need water with low to medium salinity (0-20 parts salt per thousand) to survive. The normal growth and breeding of these animals takes place in waters with this salinity range. Upon maturing the females tend to migrate back to the sounds and beaches where they find waters that are saltier for their spawn cycle.

The food source for crabs varies widely during the early stages of the life cycle. They can eat mosquito and sand gnat eggs and larva as well as the eggs and larva of grass shrimp (palaemonetes vulgaris). As the young blue crabs become juveniles their diets graduate to include fish and other animals they capture or find dead along the bottom of the estuary. It was my experience when I was crabbing that just because a blue crab is a scavenger, as are many other types of sea life, it prefers fresh food as opposed to rotten food. Ask any commercial blue crab fisherman how many crabs he catches with only rotten bait in his traps.

The experience of catching more male crabs (Jimmy's) in low salinity water as compared to water with higher salinity has taught commercial crab fishermen that male crabs prefer lower salinity water. For instance, male blue crabs have been sited many miles up the Altamaha River beyond Jesup where the water is entirely fresh. Male blue crabs have been known to bury up in the sediment in fresh water or low salinity water and weather out cold weather, but for some reason, others migrate out to the sounds or beaches as winter temperatures decrease in inland waters. During these "buried up" periods blue crabs semi-hibernate but will occasionally emerge from the sediment during warm sunny days to attempt to feed.

Many people, (including me at one time) believed that crabs were an extremely resilient and tough animal who could withstand just about any kind of water conditions but now with time and experience as a commercial crab fisherman and after listening to marine biologists and other scientists, my beliefs about the resiliency of this animal has come full circle.

Crabs by nature are cannibalistic and feed on each other as well as other marine life. They often fight for survival and get scratched or wounded in the process. Crabs have an immune system on the outside of the shell in the form of a thin slick coating. When fighting, struggling, or by severe impact from objects they can get scratched on the shell which breaks their immune system at the point of contact.

Natural bacteria in the air and the water serve many purposes, good and some times bad. A bacterium that I will describe briefly is called chitin (ky-tin) because as a layman I can only identify scant portions of what this bacterium is capable of doing. As I understand it, chitin is supposed to break down the bone and shell matter of dead animals. Chitin is an opportunistic bacterium and attacks the blue crab at the point where the immune system has been penetrated or exposed. When the bacterium attacks, the shell starts getting eaten away, at the point of penetration. Scientists call this chitinoclastic shell disease. Chitinoclastic shell disease has an ugly appearance but reportedly does not kill its host. The problem with chitin is that it exposes parts of the crab to other virus that can attack and kill the crab. Chitin also preys on other crustaceans such as shrimp.

Chitin bacteria are a natural part of the decomposition of dead animals including blue crabs but not LIVE crabs. The only time scientists find chitin attacking live crabs is in stressed waters. Stressed water means water that is low in dissolved oxygen and high in nutrients (Phosphates and Nitrates). Many scientists believe crabs already have stressed immune systems from water that I describe as polluted water. Polluted waters not only have detrimental impacts on crabs but many other forms of marine life, as well.

On Georgia's coast we have another form of pollution known as high salinities. High salinities for many in shore marine animals could be classified as greater than 25 PPT. To understand how water that is too salty can be called a pollutant you need to understand the term pollution. Any substance that alters the natural, normal condition of the water in any given area (salt water or fresh water) can be classified as a pollutant. For instance, if you have a stream that normally has a salinity value of 10-15 PPT if water invades the area with a salt contents of 25 PPT or greater, the saltier water can be classified as a pollutant. The reason for this is very simple, the saltier water can disrupt the natural functions of animals that require lower salinities for survival. Water that is too salty affects marine life in many ways, however, for the purposes of this article we will remain focused on blue crabs. In the summer months when it gets warmer, water that is too salty brings another critter called hematodinium.

The parasite hematodinium thrives in warm salty water and attacks blue crabs. It gets into the blood stream of crabs and affects the oxygen supply causing the crab to become lethargic and die for lack of oxygen. In many cases when a blue crab becomes infected with this parasite it literally becomes a death sentence. Not only is it a death sentence for the carrier of the parasite but also the parasite is transmitted into the crab that eats the infected crab. At times the infectious parasite has been found in as many as 50% of the crabs found in some of Georgia's waters. Scientists believe that nearly 100% of juvenile crabs infected with this parasite will die.

Causes for pollution in our marine waters are still being studied, however, many scientists and others believe that non point source of run off waters from municipalities, manufacturers, sub divisions, golf courses and many other things are some of the major causes of pollution from nutrients. When you look at the fact that the amount of nutrients entering our streams by human sources is nearing the amount of nutrients by natural sources, it is no wonder that we have too many nutrients in of our streams today.

Rising salinities in our streams is also a dilemma because there is little, if any, actual documented science giving us factual reasons. However, some scientists will tell you they believe the root cause for increasing salinities along Georgia's coastline is two fold:

The ditching and draining of vast numbers of coastal freshwater wetlands
The abuse of the upper floridan aquifer in coastal Georgia.

Many scientists and others believe the interaction between ground water and surface water has been severely affected by the reduction in the quantity of water in the upper Floridan aquifer.

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